The present invention relates to fluid controllers of the type used to control the flow of fluid from a source of pressurized fluid to a fluid pressure operated device, such as a vehicle steering cylinder.
A typical fluid controller of the type to which the present invention relates includes a housing which defines various fluid ports, and further includes a fluid meter, controller valving, and an arrangement for imparting follow-up movement to the valving in accordance with the flow through the fluid meter. The flow through the controller valving is directly proportional to the area of the main variable flow control orifice which, in turn, is proportional to the relative displacement between the valve members comprising the controller valving.
Fluid controllers of the type to which the present invention relates are frequently used on large, heavy vehicles. More particularly, such controllers are frequently used on articulated vehicles which have high inertia loads on the opposite side of the wheels from the pivot joint. The weight of such vehicles, and the inertia loads, have made it difficult to achieve smooth steering action, and as a result is has become common practice on such vehicles to provide a cushion valve in the lines interconnecting the fluid controller and the steering cylinder. In some systems, an accumulator is used instead of a cushion valve. One of the purposes of using either a cushion valve or an accumulator is to reduce the "lateral jerk" which occurs, for example, when the vehicle operator has been steering in one direction, then returns the steering wheel to its centered position, thus closing the controller valving. The lateral jerk occurs as a result of vehicle inertia driven pressure pulses within the system which occur because the vehicle operator is able to move the controller valving from one operating condition to another (e.g. from maximum displacement in one direction to neutral) faster than the system can deccelerate the lateral motion of the vehicle.
Although the present invention can be used with various types of controllers having various valving architectures, it is especially advantageous when used with a controller having a valving of the rotary spool-sleeve type, and will be described in connection therewith. In the typical, prior art spool-sleeve controller, the various flow orifices in the controller valving are each comprised of a plurality of ports in the sleeve overlapping a plurality of passages defined by the spool, the cumulative overlap thereof comprising a particular flow control orifice.
In conventional spool-sleeve controllers, it has been accepted practice to provide six of each of the sleeve ports and six of each of the respective spool passages. It is believed that this is done partially out of concern for proper radial balancing of the sleeve, relative to the spool. However, as a result, in the prior art spool-sleeve controller, the relative displacement between the spool and sleeve has been somewhat limited. Typically, the relative displacement between the spool and sleeve in going from neutral to maximum displacement has been only about 10 or 12 degrees, although in some larger, more recent controllers, the maximum relative displacement between the spool and sleeve is about 19 degrees.
Although the controller valving of the type described above has been generally satisfactory, it has frequently resulted in excessive lateral jerk when such controllers are used on large articulated vehicles, thus necessitating the use of a cushion valve or an accumulator in the system. The use of an accumulator has normally resulted in satisfactory performance, but an accumulator adds substantial expense to the system, and requires a substantial amount of additional maintenance. On the other hand, a cushion valve provides generally satisfactory cushioning of at least a first pressure spike, but not always of successive pressure spikes. In addition, a typical cushion valve adds substantially to the cost of the system.